A conventional system is disclosed in DE 197 39 808 A1. In this system, the code words which are transmitted from the control device to the communications interface contain an information section and an error-detection section which contains, for example, the sum of bits of the information section. The information packet contains a bit which corresponds to the presence or non-presence of a safing signal generated by a safing switch or safing sensor. If the code word transmitted by the control device is to fire firing devices of a motor vehicle occupant protection system, it contains, at a predetermined place in the information packet, a bit which corresponds to the presence of the safing signal, i.e. requires the safing switch to be closed. This information bit is replaced in the communications interface by the information relating to the safing signal which is actually present at the safing input so that, for example when there is information contained in the code word relating to the closed safing switch but when there is an absence of the safing signal at the safing input of the communications interface, the sum value in the code word no longer corresponds to the sum of the information packet and the code word can be detected as faulty, as a result of which the firing devices do not fire.
In the known system, the safing signal which is present at the safing input thus permits a safing function to be integrated into airbag firing systems which operate with bus systems. When the safing switch is opened, firing is reliably prevented. However, in the known system, it is not possible to distinguish between firing instructions for different firing devices, for example firing devices for side airbags and firing devices for front airbags. Furthermore, all the firing devices can be fired only if the safing switch is closed.